1. Field of the Invention
The present invention relates to a device for oscillation excitation of an elastic bar, which is fastened on one side in an atomic force microscope (AFM) and comprises semiconductor material, which does not have piezoelectric properties and on a free end thereof a tip is attached, which may be brought into contact with a sample surface to be studied.
2. Description of the Prior Art
With the development of atomic force microscopy (AFM), great progress has been achieved in the characterization of surface properties. For the first time, it is possible with the aid of atomic force microscopy (AFM) to obtain information about surfaces and surface-proximal areas of greatly varying samples and components at a resolution of nanometers down to individual atoms.
Atomic force microscopes are commercially available. The sensor comprises a small leaf spring of approximately 100 μm to 500 μm length having a tip which is scanned over the sample using piezoelectric actuating elements. A position sensor measures the deflection of the spring. This position sensor frequently comprises a laser diode and a position-sensitive photodiode. The laser beam is focused on the reflective rear side of the leaf spring, reflected there, and directed to the photodiode. A deflection of the spring causes a position change of the laser beam and thus a change of the photovoltage. The topography of the surface is imaged in that via a closed loop, the sensor or the sample is tracked perpendicularly to the sample surface, that is, in the z direction, during scanning in such a way that the deflection of the spring remains constant. The z voltage is coded as a color value and displayed via a computer.
Dynamic types of operation, in contrast, in which the leaf spring is set into oscillation, have increasingly gained significance in atomic force microscopy, because with their use sensitive samples may be imaged without damage and because physical properties of the sample surface may also be derived, in addition to the surface topography, from the amplitude and phase of the leaf spring oscillation.
The leaf spring is set into oscillations at or near its resonance frequency and positioned over the sample surface, so that the leaf spring only contacts the sample surface via its tip for a very short time of its oscillation period. Grinding of the tip over the sample is thus prevented, which is advantageous when studying weakly bound or soft sample surfaces. In contrast to the contactless mode, the oscillation amplitude is large enough to overcome the adhesion forces of the sample surface. Surface structures may be identified and measured using amplitude variation. If the sample tip is guided over a protrusion, for example, the amplitude of the oscillation decreases, in contrast, if it runs into a depression, the amplitude automatically rises.
With the use of so-called atomic force acoustic microscopy (AFAM), local elastic properties of materials may be imaged and quantitatively determined at a local resolution of a few nanometers. This is a dynamic AFM operating mode, in which the fact is exploited that a leaf spring oscillating at its resonance frequency may detect extremely small changes in the tip-sample interactions.
The oscillation excitation of the leaf spring is of special significance for this purpose. It is thus to be ensured that the oscillation behavior predefined by the leaf spring geometry and its intrinsic elasticity is impaired as little as possible by the means and technologies to be used in the scope of oscillation excitation.
In the following, an overview will be given of known excitation technologies of AFM leaf springs:
Because the leaf spring of an atomic force microscope, as noted at the beginning, is fastened and/or clamped on one side in a retainer, it suggests an oscillator system required for oscillation excitation of the leaf spring be integrated in the area of the retainer itself. Preferably, piezoelectric oscillator systems are used for this purpose, which are capable of also setting at least partial areas of the retainer itself into oscillation in addition to the leaf spring. A disadvantage of this type of excitation is that the natural resonance of the retainer may superimpose on the natural resonance of the elastic bar, which one wishes to measure. In addition, in this type of oscillation excitation, limits are set in regard to achievable resonance modes along the leaf spring.
Another possibility for oscillation excitation of the leaf spring comprises attaching an oscillation system directly on the leaf spring, to thus avoid interfering oscillation excitation in the area of the retainer.
It is suggested in Appl. Phys. Lett. 64, 12 (1994), J. Vac. Sci. Technol. B 15, 1506 (1997), and Appl. Phys. Lett. 85, 6398 (2004) that the leaf spring be excited using an ultrasonic transducer, in that the ultrasonic transducer is contacted with the sample to be examined on a side facing away from the sample surface. The ultrasonic transducer is connected to an external frequency generator, which supplies the transducer with a sinusoidal AC voltage, so that longitudinal and/or transverse waves are emitted into the sample and thus cause displacements perpendicular to the sample surface and/or along the surface. If the leaf spring is in contact with the sample surface via its tip, the oscillations are transmitted from the sample surface to the leaf spring, which begins to oscillate in bending and/or torsion and lateral modes. Contact resonances of the system sample-leaf spring are excited in this way, in which forces act between the tip of the leaf spring and the sample surface, due to which the resonances of the free leaf spring shift toward higher frequencies, the so-called contact resonances.
A method for measuring surface properties using AFM is disclosed in U.S. Pat. No. 6,006,593, in which the excitation of the leaf spring occurs at its suspension at the end of the leaf spring using an ultrasonic transducer which is connected to a frequency generator. This transducer transmits the oscillation to the leaf spring. If the leaf spring is in contact with the sample surface, contact resonances of the system sample-leaf spring may also be excited using this method. An ultrasonic transducer or the thermoelastic expansion of the suspension by an amplitude-modulated laser beam may be used as the transducer.
Contactless excitation technologies are also known, by which the leaf spring may be set into resonance oscillation. Thus, a method in this regard is disclosed in German Patent 103 21 931 B4, which is based on the finding that in the event of oscillation excitation of the sample surface into oscillations oriented laterally to the surface and linearly polarized along an oscillation direction as well as additional orientation of the leaf spring perpendicular to the oscillation direction over the sample surface, the leaf spring may be excited to oscillate by shear waves coupled into a gaseous coupling medium, such as air, located between the sample surface and the leaf spring.
Another approach is described by K. El Hami et al. in “Selective Excitation of the Vibration Modes of the Cantilever Spring”, Sensors and Actuators A 64 (1998), 151-155. Polymer strips made of piezoelectric material are applied along the surface of the leaf spring, which are supplied with electrical AC voltage, by which the leaf spring is set into resonant oscillations.
A semiconductor voltage sensor is described in U.S. Pat. No. 6,211,540, which provides a leaf spring clamped on one side, whose flexion is detected using a sensor, which contains a Schottky contact. If the tip of the leaf spring scans a sample surface, the leaf spring is deflected and bending of the leaf spring occurs. The Schottky contact which is located on the leaf spring changes its electrical properties because of this deformation.
Similarly to the above publication, WO 97/24915 describes a micro-electromechanical system which has a deformable structure which is combined with a sensor element, which is implemented as a Schottky contact, for example.